Assembly
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Reassembling. Before doing that, you measure to check limits. Even if the parts are new you measure to make sure the parts are within limits. To measure the case you have to put it together with specified torque but without anything in it. Keep a record of the measurements for later reference.And you have to do this messurings according to youre M/M.

Measurements are also taken on each cylinder, piston, and piston ring (to check for piston ring end-gap clearance). This is a laborious process but worth it to ensure a perfect engine. You can best keep this organized by laying out the parts on a large table with obvious tags numbering each set of parts.In this case have to use special tools for each of this activities.

Then you get out the assembly lube and start again with a naked crankshaft. Assembly lube is really gooey, but that's the point. Connecting rods go on first, with proper cotter keying for the rod bolts.Putting the case halves together and getting all of the gears and small bits included is a 5-handed operation. It might be easier to do the assembly with the case on the bench, but given an extra body doing it upright worked for us. You also have to take care to get the silk thread oil seal carefully placed on the case parting seam and avoid disturbing the seal during assembly.

Once the case is done then you install the pistons with rings and cylinders. Have a friend handy to hand you stuff while you maneuvering the piston with pin, rings, ring compressor, and the cylinder while your hands are coated with slippery goo. Once you clean up the goo, you get an upper body workout by following the torque sequence for the cylinder tie-down studs and through-stud nuts. On an O-470 you do it three times, one sequence to low torque, then to full torque, then again to make sure you got it right.

Once you get to this stage most of the visible assembly is done. There's lots of not-so-visible details to finish such as accessory drive pads, starter adapter, oil pump, tachometer drive, oil pan, oil cooler, and miscellaneous hardware like engine mount brackets, generator brackets, and the oil filler neck.

The final assembly steps are finishing the topend with pushrod tubes, pushrods, rocker arms and rocker shafts, and finally the rocker covers. I saved a few things until after painting, such as the front seal, installation of accessory pumps, and the intake manifold, but if you don't care about paint or you don't care that they're all the same color as the engine then assemble them all in one shot.

Disassembly And Cleaning
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The overhaul starts with removing the engine from the airplane. This is exciting but you'll do yourself a favor by taking time to document the installation before removal. Pay particular attention to the routing of control cables, wires, and hoses, and all of the clamps that hold those items to the engine compartment.
Before you pull the engine, drain the oil out the engine while it is still on the airplane. Since most flat aircraft engines have a wet sump, you want to minimize the amount of oil that's going to come out all over your hanger floor when you start pulling the engine itself apart. It is much better to dump only the residual quart vs. the full 7 to 10 quarts that your engine normally contains.

To get the engine off you'll usually have to remove the cowling, propeller, all of the controls and hoses, and the exhaust manifold. This is where you find out where all the dirt and grit you taxi over goes to. Further you get very familiar with any oil leaks your engine has because the oil ends up all over you.

Again you'll save yourself a lot of trouble and head-scratching later by taking the time to bag and tag all the parts as you're removing them, and to photo-document clamps, hoses, fittings, and the orientation of all of the above.

The first challenge after removal is to put the engine up onto the stand, which involves bolting the prop flange to the stand (easy) and then wrestling the engine into an upright position without dropping it during the process (tricky). Two or more people and potentially a block and tackle hoist separate from the engine crane are recommended.

Then the cylinders and pistons and finally undoing all the case bolts and splitting the case. Don't forget to remove the idler gear pin which ties the case together (doh!). As you pull the case apart lots of oil and some miscellaneous parts will fall out, so catch the ones you care about like the gears. Hopefully you won't need to use too much "persuasion" with the soft mallet for gooey drive pads and parts.

New Limits vs. Service Limits
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Overhauls (excluding for the moment factory remans) can be done to "new limits" or "service limits". Each engine design has a published table of allowable dimensions, or "limits" for the critical parts. The parts are allowed to be within a certain size tolerance when new, or "new limits". As parts wear during service then those tolerances may no longer be met, "service limit" published which define the tolerances for a part in service. If a part cannot meet the service limits then it must be repaired or discarded.

An overhaul is defined as parts being cleaned and inspected to meet service limits. It is completely legal to take a part that does not meet new limits but does meet service limits and put that part into an overhauled engine. As the owner you must decide what is acceptable,and what is cant use again.

Given the expense and disruption of an overhaul it is almost always the right choice to insist that all parts put into the overhauled engine meet new limits. This is known as a "new limits" overhaul, and it will give you a known baseline of wear that is equivalent to a new engine. In some special cases service limits may be acceptable, for example the part in question is hugely expensive to replace or repair and the limits are not far out of new.

Limits are usually defined in terms of a minimum and maximum size measured in thousands of an inch. Dimensional limits are measured with micrometers, bore gauges, calipers, and other precision measuring tools. Limits are sometimes defined as the direct measurement of a part, and sometimes as the clearance or fit between two parts.

You should consider accessory overhauls when doing your engine overhaul. Engine overhaul is probably the best time to do accessories since they're easy to get to and your airplane is offline anyway. Further the overhaul clock gets consistently reset for all of the parts in your engine compartment. A reason not to overhaul an accessory with the engine is low time in service on the accessory, perhaps due to a recent one-off accessory overhaul performed for other reasons.

Accessory overhaul is usually not included in quoted overhaul prices but can be added as an option. You can also manage the accessory overhaul yourself using accessory shops. Typically you'll need to overhaul the following
Carburetor or fuel injection system

Fuel pump
Hydraulic pump
Vacuum pump
Oil cooler
Alternator or generator
Starter
Propeller governor
Magnetos

You might need to overhaul these items as well:

Propeller
Turbos
Exhaust manifolds

For some of these items it can be more cost-effective to just buy a new unit, especially for starters due to the new "lightweight" starter that are on the market now. Another example is replacing older Bendix magnetos which have been subject to too many ADs with a set of new Slick magnetos. Engine overhaul is also a good time to do any upgrades that you've been wanting to do, such as upgrade your old generator to an alternator.

If you have a constant speed propeller you should always overhaul the governor with an engine overhaul. The governor is a high pressure oil pump and it can accumulate dirt and junk that will be pumped right back into your freshly overhauled engine if you slap your governor back on without a cleaning and overhaul.

New vs. Overhauled Engine Parts

Engine overhaul instructions usually specify a set of parts as 100% replacement parts. This includes pistons, rings, bushings, seals, gaskets, etc. For non-factory overhauls this list isn't mandatory but it is a good idea and is accepted practice. Certain parts are hard or impossible to reuse even if you wanted to, such as gaskets and O-rings.

There are also a set of parts that you reuse. These include "steel" parts such as the crankshaft, connecting rods, rocker arms, pushrods, gears and driveshafts. Overhauls also reuse the engine case, accessory case parts, and any accessory drive mounting pads.

All reused parts will be dimensionally inspected and sometimes tested with non-destructive methods for cracks or defects using techniques like Magnaflux (for steel parts) or dye-penetrant tests. Any parts that fail to meet either in-service dimensional limits or new limits if you specify a new limits overhaul will be rejected and replaced no matter what your preferences are. Unfortunately some parts come in matched sets, such as connecting rods, so replacement of one may mean replacement of all which can get to be very very expensive. Fortunately the "steel" parts rarely need replacement.

I had ECI clean, inspect, and overhaul (rebushing, polishing, plating, and installing crankshaft counterweights) all of my steel parts, the case, and accessory pads. ECI also pre-assembles complex parts such as Continental starter adapters. I did get a new camshaft and lifters because my old camshaft had some corrosion and was starting to spall off bits of metal.

A tough choice these days is whether to overhaul existing cylinders or buy new cylinders. New cylinders are relatively cheap, but they're still more expensive than overhauling your existing cylinders. If you have low wear on your existing cylinders, for example they're a first-run set that was previously installed new and has never been overhauled, then there's a good chance they can be overhauled and reinstalled. You may have to go "oversize" on the cylinder bores if they were worn beyond normal limits, but that's ok. Oversize means that the bores are machined out an extra 0.010" bigger than a new cylinders, and correspondingly bigger rings and pistons are installed. The extra material removed from the cylinder bore to get to the oversized dimension is insignificant and you get reset to a known dimension. An overhauled cylinder still gets all the important stuff replaced or freshened up, including valve guides, valve seats, cylinder barrel choke and cylinder barrel honed finish.

Of course if your engine is out of production and you cannot find new parts then you're going to be overhauling by definition. In some cases this may mean resorting to chroming cylinders and reusing parts that would normally be considered throwaways, such as pistons. Unless you're in this situation with an out of production engine, I would recommend staying away from chrome cylinders because they are somewhat unpredictable and the cost isn't justified with new cylinders being relatively cheap.

Personally for a full overhaul would always go with new cylinders. They're roughly double the price of an overhaul of your existing cylinder, but you get a completely new set of cylinders, valves, and valve hardware. For many engines you have a choice of cylinder manufacturer, ranging from factory cylinders, cylinders with through hardened or nitrided barrels, Superior all-steel cylinders with investment cast heads, ECI cylinders with steel barrels, or ECI cylinders with CermiNil barrels. Each manufacturer has their spiel and gimmicks, recommend Superior cylinders because the fit and finish is high, the steel cylinder is a time-proven configuration, and Superior parts appear to have good service life and warranty service. Continental cylinders have been subject to several recalls in recent years, and it would be incredibly annoying to have to pull apart your newly rebuilt engine to send bad cylinders back to the factory due to manufacturing defects.

Engine Installation

When your engine is off the airplane it is a good time to consider whether the engine mounting and connections are in good shape. Most of the time these items are very hard to get to, and they rarely see maintenance as a result. The only real downside of doing them during an overhaul can be the increased time and cost of replacement.

The most important item to consider is flexible hose connections for fuel, oil, vacuum, and hydraulics. Aircraft hoses have a service life limitation and they get brittle with age. The high temperature conditions which exist in engine compartments will age a host faster than if it is located in a cooler environment such as a wing. A brittle hose can break in flight without warning which can have severe consequences.

A five year service life is considered normal. Hoses can last much longer than 5 years in service, so they're not automatic throwaways, but in my book anything older than 10 years or so is suspect. Hoses are usually marked with the date of manufacture either printed on the hose itself or on metal tag attached to the hose. Since you're probably removing at least one end of all the hoses attached to the engine, it is a good time to inspect the hose dates and to flex the hoses. If the hose is hardened it is time to replace it. You'll probably be surprised how old your hoses are. I've pulled out hoses that were original to the airplane (some dating back to the 40s) and when they were flexed the rubber would literally snap in half.

If you have a set of hose mandrels you can make your own hoses for Aeroquip 303/Stratoflex 111 medium-pressure hose. This is a laborious and messy task, but it can save money. If you don't have the mandrels the chances of correct assembly are almost nil, so don't try it. If you don't have the mandrels or the patience you should send your old hoses to a hose shop for replacement. A real hose shop guarantees the hose and pressure tests the replacements. A good shop is Sacramento Sky Ranch.

Besides high-pressure hoses, the air intake hoses on most airplanes are made of "SCAT" or "SCEET" tubing which chafes and wears. If your intake hoses have holes, replace them. Each hole is potentially a place where air is bypassing the air filter or carburetor heat, which can have bad engine life or flight safety consequences respectively.

Consider whether the rubber engine mount vibration isolators are in need of replacement. Often these are replaced during engine overhaul as a matter of course. The rubber becomes cracked and hardened with age. If the rubber is hardened then it won't be a good vibration isolator.

When you go to the expense of overhauling your engine, also spend the time to make sure your air-cooling baffles are tight and are in good condition. Baffles take lots of abuse from chafing and corrosion and often need replacement or repair during engine overhauls. Don't cheap out on the few bucks it will take to replace broken or worn baffles because bad cooling will ruin the best overhaul. Even if your baffles are in good shape, always replace the baffle seals with silicon rubber seals that are cut to the proper length to make a good seal with your airplane's cowling.

Also don't forget to budget for removal and reinstallation of the engine. This can be a time consuming task, and depending on the aircraft you should consider 10 to 20 hours as a minimum.

Kind Of Overhaul
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When overhauling an engine, there are four basic choices for the kind of overhaul you want:

Choice 1: Factory Remanufactured Engine. This strictly speaking isn't an overhaul, as it results in a zero-timed engine. You send your run out engine "core" to the factory, either Continental or Lycoming, and you get back an engine that is built to new engine standards. Some parts are re-used such as crankshafts and cases, but top end parts are generally replaced with new parts. Since the factory does the work, they can certify the engine as "zero time" and it counts as a brand new engine as far as the logbook times are concerned.

Here's the Lycoming propaganda on factory remains.

This option is the gold standard for higher-end and more expensive aircraft such as late model Bonanzas and Cessna 210s and twins, where the presence of a factory reman has a noticeable affect on the aircraft's overall price and salability. A factory reman is also the most expensive option with least flexibility in terms of options and finish.

If you own one of these more expensive aircraft, and you plan to keep it for several years, then a factory reman is probably the best choice when your engine is run out.

Choice 2: Factory Overhauled Engine. The factory overhauls the engine, but the engine is not zero timed. In theory a modern factory overhaul, where the factories have begun using more new parts, will result in exactly the same engine as a reman. The key differences are price and lack of a zero-timed fresh logbook, and potentially less risky warranty coverage because the warranty is backed by the factory.

I think factory overhauls are hardly worth it because you pay a premium, don't get a reman's patina of goodness, and you don't get a lot of flexibility in finish. If you're looking for a non-reman overhaul, I'd go with a name-brand shop.

One downside of factory overhauls and remans is that you generally won't get your own engine parts back. You'll get somebody else's used case and crankshaft. If you know yours are no good, this can be advantageous because the factory will replace your worn-out parts, but if you have good parts then you may want to keep them because you know the service history (since you flew them). A factory reman or overhaul does not give you the option to keep your original parts. You may or may not care.

Choice 3: "Name Brand" Overhaul Shop. There are a number of "name brand" overhaul shops that have enough volume to justify national advertising for their services. These shops are specialists in engine overhauls and should be the best experts you can find outside of the factory - some would argue they know more than those inside the factory. Examples include Western Skyways, Mattituck, Penn Yan Aero, and Victor Aviation.

The cheapest overhaul from a name-brand shop is nowadays about the same price as a factory overhaul because the factories have been aggressively competing for overhaul business. The custom shops usually offer value-added services and customization flexibility that the factory does not offer. Examples include in-house accessory overhaul, customized matching of parts by weight or fit to tolerances that are tighter than factory tolerances, dynamic balancing, specialized paint schemes, and use of non-OEM parts sources such as Superior Air Parts. If any of these services are important to you, or you want to have more control over how your engine is built up, then a name brand overhauler is a good choice.

Choice 4: Field Overhaul. Any certified Powerplant mechanic with Inspection Authority can perform and sign off an engine overhaul. The mechanic may or may not be associated with a FBO, although most are. A field overhaul is often performed locally and so you have the most knowledge of the mechanic, and potentially the most control over the schedule and overhaul options ranging from a completely vanilla service-limits overhaul up to a completely tricked out custom built and finished engine. It is also potentially the most work for you as owner and is probably the least valued by the marketplace for resale value because buyers of your aircraft usually will not have the same knowledge of and trust in your mechanic that you do.

A field overhauler will almost never have direct access to specialized overhaul tooling and equipment such as cylinder boring and honing machines or valve seat grinders. In practice this is not an issue because with there are specialty parts overhauler shops that make a business of doing this kind of work and your mechanic will simply send the parts out.

The advantages of a field overhaul are price and flexibility, but the disadvantages are potentially lack of expertise, lack of longevity and reliability if you have a warranty claim, and lack of reputation or consistency which can especially impact resale value. You have to decide how well you know your mechanic and what his or her record is on overhauls and customer service.

Decision Time

You can get the same quality of engine from each type of overhaul. Your preferences, schedule flexibility, cash flexibility, and personal knowledge of the overhauler's reputation should drive the choice. A safest choice, albeit highest-dollar choice, is a factory reman, and if you have a high-dollar aircraft I would always go with the factory reman in order to support the resale value and the salability of the aircraft. A name-brand overhauler will do a consistently good job. If you are choosing between a factory or name-brand independent overhauler I personally would go with the name-brand overhauler, because you'll probably get better service and you'll definitely have better flexibility with the name-brand shops.

To be honest the real reason I did a field overhaul was to get the experience of performing the overhaul myself, working under the supervision of Rob. This option is not for everybody, but it really doesn't take any special skills and you learn a tremendous amount about your engine. I urge folks to give it a try because you'll end up with outstanding knowledge of how your engine works and the confidence of having put it together yourself.

A few common questions that folks ask about doing an overhaul yourself:

Do you have to be an FAA-certified mechanic? No, you do not, but you must work under the supervision of a certified mechanic. Further, that mechanic must trust your work enough to put his name and license on the line since as far as the Feds are concerned, whoever signs the logbook did the overhaul.

You need to find the right kind of mechanic to work with, because not all mechanics are willing to have you do the work. You should also have a good working relationship with your mechanic, because you're going to spend a fair amount of time working together and you're probably going to be asking a lot of dumb questions.

How long will it take? The answer is really determined by how much time are you going to put into the overhaul. There is a fixed amount of calendar time that will be consumed by the parts overhaulers, and you most likely cannot change that time. The time spent disassembling, inspecting and measuring, and reassembling, is the time that you can control. Working 8 hour days, with all parts, supplies, and tools on-hand, it is possible to do the job in 3-5 days. Most likely you'll take longer because you don't already know the procedures, because life at the airport is usually full of interruptions, and because of logistical delays for parts and supplies.

From starting to remove the engine from the aircraft, to finish of the first flight, my overhaul took 7 months of calendar time. This was a very relaxed pace averaging about two full weekends of work per month. The work included custom painting and overhauling of the engine mount, baffles, and airbox, and it included about 5 weeks of time used by the parts and accessory overhaul shops.

Will it cost significantly more or less than a "normal" overhaul? The cost delta between a professional overhaul vs. doing the work yourself are largely going to be in labor. This is a subject to discuss with your mechanic before beginning the overhaul, since your mechanic may spend more time with you than he would if he was working alone. You can also save a significant amount of labor time if you work alone since presumably your time is "free".

Are there special skills involved? There are no special skills that are required. Knowledge of hand tools and mechanical work is helpful but not required. Aircraft engines are really quite simple (by design) and the procedures for assembly are reasonably well documented. You will need some special tools beyond standard hand tools, but you can probably use your mechanic's tools if you respect your mechanic's boundaries and tools (you should talk about this out beforehand).

Is it safe? The real question here is do you trust yourself? Are you careful, do you follow procedures and checklists, and if there's a possibility of mistake are you willing to undo your work to verify that the assembly is correct? I personally trust my work more than some anonymous mechanic who I've never met and who will not be flying the aircraft.

Time For An Overhaul
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In late 2000 it became obvious that my Bellanca was in need of an engine overhaul.
The engine type is a Continental O-470-K horizontally opposed "flat" six cylinder air-cooled aircraft engine. The engine was running ok, but the compression, which is a good indicator of the health of an engine, was sagging into the 50's (a perfect score is 80, and 60 is generally considered the lowest allowable value). The engine had about 1200 hours total time, which was getting to be "high time" since the recommended time between overhaul (TBO) for an O-470-K is 1500 hours.

In many cases this would be a situation where a top overhaul would be appropriate. A top overhaul is where you remove and overhaul the "top end" of the engine: cylinders, pistons, and valves. During a top you do not disassemble or overhaul the "bottom end" of the engine which consists of the connecting rods, crankshaft, camshaft, engine case, and accessory gear train and drive pads.

A top overhaul would be appropriate where the condition of the bottom end of the engine is known to be good, and you want to squeeze out some more operation before going in for a full overhaul at the recommended TBO interval. Tops are relatively cheap and don't take as much time. Tops are often performed on big-bore Continental engines (of which the O-470 is one) at mid-life, so it wouldn't be unheard of. Getting 300 or so more flying hours, which is 3-5 years of flying for me, for a few thousand dollars is a reasonable investment.

I decided to go with a full, or "major" overhaul, because of the calendar age of the engine. The airplane was built in 1958 and in late 2000 had relatively low time for the calendar age of the airframe (1200 hours). The logbooks were spotty, so it was impossible to tell for sure, but it was highly likely that the engine had never been disassembled since it left the factory. This assumption was borne out by older hardware and many leaking oil seals where the rubber had hardened.

Given the calendar age of the engine, and given a long interval that the engine sat unused during the 1970s, I decided the best course was to do a major overhaul instead of a top. A top would probably have fixed the sagging compression, but it wouldn't have addressed any internal issues such as camshaft corrosion, and further it wouldn't have addressed age issues such as the cylinder casting styles being long out of date and hard-to-fix oil leaks due to old and cracked seals.

Since I'm something of a mechanical perfectionist, it wasn't that difficult to take the plunge into a full overhaul. Once decided on the course, I then needed to decide who was going to overhaul the engine.

Engine Overhaul Terminology and Standards
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TBO time draws near and you need to decide on a course of action. You call around, talk to your maintenance facility, and find that you are totally confused because you don't understand or know the definitions of many of the terms used by the people that you have been talking to.

New limits, Service limits, Remanufactured, Rebuilt, New, Used, Overhauled, Like new, OEM, Aftermarket, what does it all mean?

Lets look at and define the terms that are approved to be used by the FAA.
A NEW ENGINE is an engine that has been manufactured from all new parts and tested by an FAA approved manufacturer. The engine will have no operating history except for test cell time when received. No FAA approved manufacturer can approve another entity to manufacture or assemble a NEW ENGINE.

NEW LIMITS are the FAA approved fits and tolerances that a new engine is manufactured to. This may be accomplished using standard or approved undersized and oversized tolerances.

SERVICE LIMITS are the FAA approved allowable wear fits and tolerances that a new limit part may deteriorate to and still be a useable component. This may also be accomplished using standard and approved undersized and oversized tolerances.
An OVERHAULED ENGINE is an engine which has been disassembled, cleaned, inspected, repaired as necessary and tested using FAA approved procedures. The engine may be OVERHAULED to NEW LIMITS or SERVICE LIMITS and still be considered a FAA approved OVERHAUL. The engine's previous operating history is maintained and it is returned to you with zero time since major OVERHAUL and a total time since new that is the same as before the OVERHAUL.

A REBUILT ENGINE is an engine that has been OVERHAULED using new and used parts to NEW LIMITS by the manufacturer or an entity approved by the manufacturer. At the current time neither Teledyne Continental nor Textron Lycoming approve any other entity to REBUILD engines for them. The engine's previous operating history is eradicated and it comes to you with zero hours total time in service, even though the engine may have had used components installed that have many hours of previous operating history . For years Textron Lycoming used the term Remanufactured in their advertising and commercial media to describe their factory rebuilt engines. Around 2001, Lycoming discontinued the use of the term remanufactured and started using the term REBUILT to describe their factory rebuilt engines. So, prior to 2001 when Lycoming used the term Remanufactured to describe an engine, that engine should be considered to be a REBUILT engine

When an engine is OVERHAULED or REBUILT the new parts that are used during the repair process can come from a variety of sources. An O.E.M. part is a new part that is manufactured by the original engine manufacturer to stringent F.A.A. standards. An AFTERMARKET part is a new part that is manufactured by someone other than the original engine manufacturer that meets or exceeds the same stringent F.A.A. guidelines as a new O.E.M. part.

Any other terms used to describe the work performed during a engine overhaul are defined by the person or entity using them. They have no official meaning and often times are very misleading. Terms like "overhauled to factory specs or tolerances", "rebuilt equivalent", "overhauled to like new condition" and "remanufactured to factory fits and limits" and any other terminology that isn't defined above needs to be investigated as to what those terms actually mean. You will probably find that advertisements and log entries that use undefined terminology are not really delivering what you think you are getting. There are specific requirements by the FAA for the use of the terms OVERHAULED and REBUILT in an engine's maintenance records. If these requirements are not met it is illegal to use the terms. Any terms other than those listed have no meaning in the eyes of the FAA and should not be accepted by you in your engine log books.

Now that we understand all the terms, let's put it all in a nut shell. Only the manufacturer can currently produce a new or rebuilt engine. Both new and rebuilt engines are made to new limits. A new engine will have all new O.E.M. parts. A rebuilt engine can be produced using a combination of used and new O.E.M. parts. An overhauled engine can be done to new limits or to service limits or a combination of the two using used parts and new O.E.M or new aftermarket parts. An overhauled engine comes to you with it's previous operating history intact and zero hours since major overhaul. A new or rebuilt engine comes to you with no previous operating history and zero hours time in service, even though, in the case of a rebuilt engine, some of the parts used may have a previous operating history.
Understanding these terms and the regulations that apply to them, may make the decisions that you have to make, at TBO time, a little easier.

TBO :- One important measure of an aircraft engine's overall economics is how often it has to be overhauled, the so-called time between overhaul, typically seen as TBO or TBOH. The TBO is a time 'recommended' by the manufacturer and depending upon how the aircraft is being operated.

A New Engine :- is an engine that has been manufactured from all new parts and tested by an FAA approved manufacturer.

New Limits :- are the FAA approved fits and tolerances that a new engine is manufactured to. This may be accomplished using standard or approved undersized and oversized tolerances.

Overhauled Engine :- is an engine which has been disassembled, cleaned, inspected, repaired as necessary and tested using FAA approved procedures.

TSMO :- Time Since Major Overhaul Amount of time accrued since the Major Overhaul.

Total Time [ TT ] :- Total Time Accumulated on the component.

Tach Time :- Time in hours recorded on the Tachometer.

A Rebuilt Engine :- is an engine that has been overhauled using new and used parts to new limits by the manufacturer or an entity approved by the manufacturer.

Aftermarket Part :- is a new part that is manufactured by someone other than the original engine manufacturer.

Manufacture Minmum-Maximum :- Dimensions and clearances that are considered to be factory new.

General
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There comes a time in the life of every piston aircraft when its engines must be overhauled. There's no exception to this rule. Based on their TBO (Time Between Overhaul) ratings, every piston engine must be removed, taken apart, checked, serviced, have components repaired or replaced as needed, and then be reassembled and reinstalled by trained professionals in order to remain in service.

Piston engine TBO is predominantly predetermined by their manufacturers based on a combination of their hours of service and their age. Each engine manufacturer has their own specific requirements, as far as what has to be done to an engine, in order to qualify as an engine overhaul.

The major purpose of overhaul is to inspect the engine parts. Inspection is the most precise and the most important phase of the overhaul. Inspection cannot be slighted or performed in a careless or incomplete manner.

Tools And Equipments For An Overhaul

Tools And Equipments
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An engine overhaul does require some special tools. These tools are used for measuring clearances as well as special assembly or disassembly procedures unique to the engine.
Here's a list of tools that you might need. The list will vary for different types of engines. For tools that are unique to the engine, you should consult the engine overhaul manual. Actually you need two types of tools and those two types are,
(A). General Hand Tools,
(B). Special Tools,
Now lest see through all this tools which are in section (A) and which are in section (B),
(A). General Hand Tools :- You'll need the following,

Short-pattern "stubby" box-end wrench set

Long-pattern box-end wrench set

60 degree double-offset box-end short-pattern wrench set

X-Acto knife, angle blades and flat blades for gasket scraping.

Cotter key puller tool

Duckbill and needlenose pliers

Small set of snips

shallow socket set and drivers, with extension set

Assorted screwdrivers

Preparation For Overhaul

Preparation For Overhaul
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Before start the overhaul according to the manual there is some action to be taken, and these actions are known as some primary preparations before an overhaul. Now lets take a look at about the actions mentioned before,
Clean oil, grease, dirt and dust from transmission.

Remove the drain plug to drain oil. Tighten the engine oil drain plug after draining.

Rotating parts should be coated with oil prior to assembly.

All disassembled parts, if to be reused, should be reinstalled in the original positions and directions.

Always use new ones for gaskets, lock washers and lock nut.

Liquid gasket should be applied where specified to prevent leakage.

What Is An Engine Overhaul

Definition And Types Of Overhaul
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An engine overhaul means, Process of restoring and maintaining an equipment, machine, or system in a serviceable condition. Overhaul involves (1) partial or complete disassembly of the item, (2) inspection to detect damaged, defective, or worn parts, (3) repair or replacement of such parts, and (4) reassembly, testing, and trial-run prior to returning the item to its full operating level.
The term engine overhaul has two identifications that make a distinction between the degrees of work on an engine,
 Major Overhaul :- A major overhaul consists of the complete disassembly of an engine. The overhaul facility inspects the engine, repairs it as necessary, reassembles, tests, and approves it for return to service within the fits and limits specified by the manufacturer’s overhaul data. This could be to new fits and limits or serviceable limits.
 Top Overhaul :- Top overhaul consists of repair to parts outside of the crankcase, and can be accomplished without completely disassembling the entire engine. It can include the removal of cylinders, inspection and repair to cylinders, inspection and repair to cylinder walls, pistons, valve-operation mechanisms, valve guides, valve seats, and the replacement of piston and piston rings.

Aircraft Engine Overhaul

Why Overhaul Is So Important
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Now a days our expensive aircraft engines, the trusty metal hearts that pull (or push) us airborne and transport us over the mountains, rivers, and weather to our destinations, don't like to be ignored, and also remember engine is the beating heart of the aircraft. They like a lot of action. They like to fly. They're most likely to provide efficient service if you fly them every day.
One engine manufacturer uses 40 hours a month for extended warranty coverage adjustment. When engines are operated at this frequency the engine life-shortening problems attributed to inactivity become a nonissue. Those engine life-shortening factors are acids, water, and lack of lubrication.

In a perfect world, piston engines wouldn't need overhauls. And if they did, their TBO periods would be far longer than 2000 hours.


But, aircraft engines function in a real world where the environment is far from friendly. In fact, "The environmental conditions that an engine is operated in play a major role in causing wear-and-tear, be it corrosive salt water air; a dry, dusty environment, or one where the climatic changes are dramatic."In such conditions, the result can be excessive internal corrosion."


Short-term use is also no favor to piston engines. The reason: "Short term use results in increased moisture condensation within the engine," said Sloan. In other words, unless a piston engine is run for reasonable periods of time (an average of 40 hours/month is recommended) it will experience corrosion due to condensation that doesn't get evaporated off its parts due to heat.Besides the external environment, piston engines experience a great deal of expansion and contraction due to the heat of internal combustion, and the cool-down after landing. The more metal is expanded and contracted, the more prone it is to wear and degradation. Add the metal-against-metal friction caused every time a piston engine fires up -- a problem that can be minimized by proper lubrication, but not eliminated -- and wear-and-tear is inevitable.


In this real world of heat, moisture, and dirt, piston engine components take a beating. "The most common parts that need replacement are cylinders, "Connecting rod bolts are another one of those items that need to be replaced." This is why regular overhauls are so important; not just to maximize engine life, but to ensure enough power for the safe and reliable operation of the aircraft in all flying conditions.


Once we summarized this whole story in to a one paragraph we can say, both maintenance and overhaul operations are performed on aircraft engines at specific intervals. This interval is usually governed by the number of hours the engine has been in operation. Test and experience have shown in operation beyond this period of time will be inefficient and even dangerous because certain parts will be worn beyond their safe limits. For an overhauled engine to be as a new one, worn parts as well as damage parts must be detected and replaced during overhaul.